An SSD (Solid State Drive) used as an external memory device of a host apparatus employs a nonvolatile memory, such as an NAND-type flash memory, which needs an erase operation at the time of rewriting data. It has been known that, in a case where such nonvolatile memories are used, copying of a large amount of data is performed inside an SSD at the time of execution of a rewrite request from a host apparatus. The life and the writing performance of a product are substantially affected by how large or small the amount of such copying is.